1. Field of the Invention
The present invention relates to a polishing apparatus for polishing a workpiece such as a semiconductor wafer to a flat mirror finish, and more particularly to a rotary-type polishing apparatus which allows a polishing pad to be automatically replaced without stopping rotary or circulatory motion of a polishing table.
2. Description of the Related Art
Recent rapid progress in semiconductor device integration demands smaller and smaller wiring patterns or interconnections and also narrower spaces between interconnections which connect active areas. One of processes available for forming such interconnections is photolithography. Though a photolithographic process can form interconnections that are at most 0.5 μm wide, it requires that surfaces on which pattern images are to be focused by a stepper be as flat as possible because depth of focus of an optical system is relatively small.
It is therefore necessary to make surfaces of semiconductor wafers flat for photolithography. One customary way of flattening surfaces of semiconductor wafers is to polish them with a polishing apparatus, and such a process is called Chemical Mechanical Polishing (CMP) in which semiconductor wafers are chemically and mechanically polished while supplying a polishing liquid comprising abrasive grains and chemical solution such as alkaline solution.
In a manufacturing process of a semiconductor device, a thin film is formed on a semiconductor device, and then micromachining processes, such as patterning or forming holes, are performed thereon. Thereafter, the abov processes are repeated to form thin films on the semiconductor device. Recently, semiconductor devices have become more integrat d, and structure of semiconductor elements has become more complicated. In addition, the number of layers in multilayer interconnections used for a logical system has been increased. Therefore, irregularities on a surface of a semiconductor device are increased, so that step height on the surface of the semiconductor device becomes larger.
When irregularities of a surface of a semiconductor device are increased, the following problems arise. Thickness of a film formed in a portion having a step is relatively small. An open circuit is caused by disconnection of interconnections, or a short circuit is caused by insufficient insulation between layers. As a result, good products cannot be obtained, and yield is lowered. Further, even if a semiconductor device initially works normally, reliability of the semiconductor device is lowered after a long-term use.
Thus, during a manufacturing process of a semiconductor device, it is increasingly important to planarize a surface of the semiconductor device. The most important one of planarizing technologies is chemical mechanical polishing (CMP). During chemical mechanical polishing, a polishing apparatus is employed. While a polishing liquid containing abrasive particles such as silica (SiO2) therein is supplied onto a polishing surface such as a polishing pad, a substrate such a semiconductor wafer is brought into sliding contact with the polishing surface, so that the substrate is polished.
FIGS. 16 and 17 of the accompanying drawings show a conventional polishing apparatus for carrying out a CMP process. As shown in FIGS. 16 and 17, the conventional polishing apparatus comprises a polishing table 102 having a polishing pad (polishing cloth) 100 attached to its upper surface, a motor 104 for rotating the polishing table 102, and a vertically movable top ring 106 for holding a substrate W such as a semiconductor wafer with its surface, to be polished, facing the polishing pad 100. While the polishing table 102 and the top ring 106 ar being rotated independently about their own axes, the substrate W is pressed against the polishing pad 100 under a constant pressure by the top ring 106, and a polishing liquid is supplied from a nozzle (not shown) to the polishing pad 100, thereby polishing the surface of the substrate W to a flat mirror finish. The polishing liquid comprises fine abrasive particles of silica or the like suspended in an alkaline solution or the like. The substrate W is polished by a chemical mechanical polishing action which is a combination of a chemical polishing action performed by the alkaline solution and a mechanical polishing action performed by the abrasive particles of silica or the like.
The polishing pad 100 is usually regenerated by a dresser which comprises a nylon brush, diamond particles, or the like. When the polishing pad 100 is worn to an extent that its polishing capability can no longer be restored by the dresser, the polishing pad 100 is replaced with a new one.
The polishing pad 100 is generally attached to an upper surface of the polishing table 102 by an adhesive tape. For replacing the polishing pad 100 with a new one, it is necessary to temporarily stop a CMP process, and a skilled operator is required to peel off the polishing pad 100 and attach a new polishing pad 100 to the polishing table 102.
FIG. 18 of the accompanying drawings shows another conventional polishing apparatus for eliminating the above drawbacks. The polishing apparatus shown in FIG. 18 has a polishing pad 100 attached to a polishing table 102 under vacuum developed by a vacuum attraction section 108 provided in the polishing table 102. Since the polishing pad 100 is removed from the polishing table 102 by releasing the vacuum, the polishing pad 100 can easily and quickly be replaced with a new one. However, replacing the polishing pad 100 requires temporarily stopping a CMP process because the polishing pad cannot b replac d while the polishing pad table 102 is rotating.
Still another conventional polishing apparatus is shown in FIG. 19 of th accompanying drawings. In FIG. 19, a polishing table 110 is fixed in position, and a pair of rolls 112, 114 are rotatably disposed one on each side of the polishing table 110. An elongate polishing pad 116 wound onto the roll 112 is continuously fed at a constant speed along an upper surface of the polishing table 110, and beneath a substrate W, toward the other roll 114 onto which the polishing pad 116 is wound. The substrate W is polished by the elongate polishing pad 116 as the polishing pad travels over the polishing table 110 in one direction. Principles of the polishing apparatus shown in FIG. 19 are not applicable to a rotary-type polishing apparatus in which a polishing table makes rotary or circulatory motion.